Add MHD support to the initial-value solver

[akelly810]

Currently the IVP state vector supports isothermal-1d, hd-1d, and hd physics types. Adding MHD just requires hooking up the magnetic vector potential components (a1, a2, a3) which are already declared but not yet wired to initial conditions.

Tasks:

• Add ic_magnetic_t (or reuse ic_velocity_t) to mod_iv_initial_conditions.f08 for a1, a2, a3
• Add setter routines set_ic_vector_potential_[1/2/3]_funcs
• Add mhd case to the select case in mod_iv_state_vector.f08
• Add a unit test in mod_test_iv_state_vector.pf
• Update the tutorial page